Biology Reference
In-Depth Information
also functioned in
D. melanogaster
after
P
element-mediated transformation,
although some regulatory interactions had diversified (
Mitsialis et al. 1989
).
4.11.2 Insecticide Resistance
Geneticists have identified a new mechanism by which insects become resis-
tant to pesticides (
Mouches et al. 1990, Devonshire and Field 1991, Pasteur and
Raymond 1996, Hemingway et al. 1998, Field 2000, Paton et al. 2000, Puinean
et al. 2010
). Amplification of esterase genes in the aphid
Myzus persicae
and the
mosquito
Culex pipiens quinquefasciatus
results in identical gene copies present
in tandem arrays in each cell.
Whether exposure to pesticides can induce resistance in insects by gene ampli-
fication is an interesting question. It has long been assumed that pesticide resis-
tance in insects is due to the presence of rare alleles in populations that are
selected for by pesticide applications (preadaptive mutations). However, amplifi-
cation of genes in mammalian cells, plants, yeast, and microorganisms has been
shown to occur in response to exposure to toxins (
Stark and Wahl 1984
). For
example, amplification of the dihydrofolate reductase gene in mammalian cells
in tissue culture occurs in response to exposure to the cancer drug methotrexate.
A 100-fold amplification in a cholinesterase gene in two generations of a human
family subjected to prolonged exposure to parathion was demonstrated and
could be due to genetic changes induced by prolonged exposure to this insecti-
cide (
Prody et al. 1989
). Mosquito cells selected with methotrexate also developed
amplified genes (
Fallon 1984
). Thus, insecticide resistances due to gene amplifica-
tion in insects could, at least in some cases, be induced by exposure to insecticides.
Resistance to neonicotinoids in the aphid
Myzus persicae
was investigated
using microarrays containing all known detoxification genes. Overexpression
(22-fold) of a single P450 gene was due to gene amplification (for a description
of microarrays, see Chapter 6) (
Puinean et al. 2010
). The microarray analysis also
showed overexpression of cuticular protein genes (2- to 16-fold), and penetra-
tion assays using radiolabeled insecticide indicated reduced cuticular penetra-
tion also contributed to the resistance.
4.12 Multiple Genomes in or on Insects: What is the “Biological
Individual”?
Eukaryotes have a nuclear genome and a mitochondrial genome. Mitochondria
are now generally accepted as microbial
symbionts
that were modified after
a long process of evolution within eukaryotic cells (
Gray 1989, Martin 1999
).
Mitochondria retain a distinctive genome that is replicated and expressed, but
